The present invention relates to band-pass signal filtering, and more particularly to selectable band-pass filtering apparatus for use with spectrum analyzers for testing multi-channel communication systems.
Communication systems such as CATV or wireless communication systems use a multiplicity of signals that share a common physical link. In particular, CATV systems typically use up to 100 or 120 carriers on a same optical fiber or coaxial cable. Some of the instruments used to test these systems are of a wideband type, such as the spectrum analyzer, which is a broadband sweeping receiver. In order to accommodate the wideband fast sweep, the input circuitry needs also to be of a wideband type, with the result that incoming signals generate distortion products within the analyzer due to non-linear characteristics thereof, which distortion products increase with input power. These inherent distortion products being undistinguished from signal anomalies generated by the communication system under test, reliability of wideband spectrum analysis is therefore limited.
However, for some tests, the wideband capability of the spectrum analyzer is not required, because only a small portion of the available spectrum need to be analyzed. Therefore, it is usually convenient to restrict the frequency band of the signal fed to the instrument with appropriate filtering, hence minimizing input power and consequently distortion products within the instrument. Since the major distortion products are of the second and third order, these distortion products are respectively proportional to the second and third power of the incoming signals, and consequently, a reduction in input power of 1 dB increases the dynamic range between signals and associated distortion products by 1 dB and 2 dB, respectively. Several approaches have been heretofore proposed to implement such filtering. A first one uses a plurality of fixed band-pass filters that may be packaged separately or a few in a same casing, each band-pass filter having preset center-frequency and bandwidth characteristics corresponding to a single or a few communication channels. In use, the operator has to select a proper band-pass filter for the current test. A main drawback with fixed band-pass filters is that the center-frequency and band-width characteristics of each filter cannot be adjusted. A second known approach uses a tunable filter, which generally consists of a selection of band-pass filters each being tunable over a wider band such that they slightly overlap from the low end of one to the high end of the other, so that the user can select any desired channel. The control device for such tunable filter may be either mechanical or electronic. Known tunable filters available in the market place are supplied from Trilithic inc. (Indianapolis, Ind.) and KandL Microwave Inc. (Salisbury, Md.). Another type of tunable filter combines a low-pass filter section with a high-pass filter section, wherein one or both of those filter sections are made variable using different design techniques. Such tunable band-pass filter for a CATV converter is disclosed in U.S. Pat. No. 4,849,721 issued on Jul. 18, 1989 to Matsuura, which band-pass filter includes a continuously variable low-pass filter section using a variable trap analog circuit. Another tunable band-pass filter for signal noise reduction is described in U.S. Pat. No. 4,894,566 issued on Jan. 16, 1990 to Rush, which filter includes a variable high pass filter in series with a variable low-pass filter in an adaptive configuration. Although providing more flexibility for setting band-pass filter characteristics as compared to fixed band-pass filters, tunable filters are generally bulky and expensive, and therefore are not appropriate for use as a field test equipment.
It is therefore a main object of the present invention to provide a selectable band-pass filtering apparatus and method of a simple and economic design, featuring improved flexibility for setting band-pass filtering characteristics.
It is a further object of the present invention to provide a selectable band-pass filtering apparatus for use with a spectrum analyzer for testing a multi-channel communication system, featuring improved flexibility for setting band-pass filtering characteristics at the analyzer input.
It is a still further object of the present invention to provide a selectable band-pass filtering spectrum analyzer system for testing a multi-channel communication system, featuring improved flexibility for setting band-pass filtering characteristics at the input thereof.
According to the above main object, from a broad aspect of the invention, there is provided a selectable band-pass filtering apparatus comprising an input end for receiving an electrical signal to be filtered, a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum and a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding low-pass cut-off frequency within the input frequency spectrum. The apparatus further comprises an output end, switch means capable of connecting a selected one of said high-pass filters in series with a selected one of said low-pass filters between the input and output ends to select a passband having center frequency and bandwidth characteristics defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency, said connected filters generating a corresponding band-pass filtered signal at the output end, and a control device capable of operating said switch means according to said selected filters.
According to further broad aspect of the invention, there is provided a selectable band-pass filtering apparatus for use with a spectrum analyzer for testing a multi-channel communication system. The apparatus comprises an input end for receiving a communication signal to be filtered, a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum characterizing the communication system, and a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding low-pass cut-off frequency within the input frequency spectrum. The apparatus further comprises an output end to be coupled to an input of the spectrum analyzer, switch means capable of connecting a selected one of said high-pass filters in series with a selected one of said low-pass filters between said input and output ends to select a passband having center frequency and bandwidth characteristics being defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency, said selected passband characteristics corresponding to one or more selected channels used by the communication system, said connected filters generating a band-pass filtered communication signal corresponding to the selected channels at the output end, and a control device capable of operating said switch means according to the selected filters.
According to a still further broad aspect of the invention, there is provided a selectable band-pass filtering spectrum analyzer system for testing a multi-channel communication system. The spectrum analyzer system comprises a selectable band-pass filtering unit including an input end for receiving a communication signal to be filtered, a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum characterizing the communication system and a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding high-pass cut-off frequency within the input frequency spectrum. The filtering unit further includes an output end, switch means capable of connecting a selected one of said high-pass filters in series with a selected one of said low-pass filters between the input and output ends to select a passband having center frequency and bandwidth characteristics defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency, said selected passband characteristics corresponding to one or more selected channels used by the communication system, said connected filters generating a band-pass filtered communication signal corresponding to the selected channels at the output end, and a control device capable of operating said switch means according to the selected filters. The spectrum analyzer system further comprises a spectrum analyzer unit for analyzing the filtered communication signal.
According to a still further broad aspect of the invention, there is provided a method for selectively band-pass filtering an electrical signal, said method comprising the step of: providing a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum; providing a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding low-pass cut-off frequency within the input frequency spectrum; and directing said signal through a selected one of said high-pass filters and through a selected one of said low-pass filters to generate a band-pass filtered signal having frequency corresponding to a passband having center frequency and bandwidth characteristics defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency.
According to a still further broad aspect of the invention, there is provided a method for selectively band-pass filtering a communication signal to be fed to a spectrum analyzer for testing a multi-channel communication system. The method comprises the steps of: providing a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum characterizing said communication system; providing a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding low-pass cut-off frequency within the input frequency spectrum; and directing said signal through a selected one of said high-pass filters and through a selected one of said low-pass filters to generate a band-pass filtered communication signal corresponding to one or more selected channels and having frequency characteristics corresponding to a selected passband having center frequency and bandwidth characteristics defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency.
According to a still further broad aspect of the invention, there is provided a method for spectrally analyzing a communication signal supplied by a multi-channel communication system. The method comprises the steps of: providing a high-pass filter section including a plurality of separate high-pass filters each characterized by a respective high-pass cut-off frequency within a predetermined input frequency spectrum characterizing the communication system; providing a low-pass filter section including a plurality of separate low-pass filters each characterized by a corresponding low-pass cut-off frequency within the input frequency spectrum; directing said signal through a selected one of said high-pass filters and through a selected one of said low-pass filters to generate a band-pass filtered communication signal corresponding to one or more selected channels and having frequency characteristics corresponding to a selected passband having center-frequency and bandwidth characteristics defined by corresponding said high-pass cut-off frequency and said low-pass cut-off frequency; and spectrally analyzing the band-pass filtered communication signal.